releasetools: ota_from_target_files uses target_files_diff.

The dependency was overlooked as the module was conditionally imported.

Bug: 63866463
Test: `m -j ota_from_target_files`; Run ota_from_target_files with
      `--log_diff` flag (on a non-A/B target).
Change-Id: I71eff2e55cc28fd649f0ca1b38da53907dac6597
diff --git a/tools/releasetools/Android.bp b/tools/releasetools/Android.bp
index 80c233b..3732b78 100644
--- a/tools/releasetools/Android.bp
+++ b/tools/releasetools/Android.bp
@@ -144,7 +144,9 @@
     srcs: [
         "edify_generator.py",
         "ota_from_target_files.py",
+        "target_files_diff.py",
     ],
+    main: "ota_from_target_files.py",
     libs: [
         "releasetools_common",
         "releasetools_verity_utils",